Following the attack in Salisbury Street, Marylebone, on January 8 this year, the teenage killer fled the scene.

He later handed himself into police at Paddington Green Police Station.

During the nine-day trial at the Old Bailey the defendant claimed he was a friend of Bilal, a student at Westminster College, who planned to rob him of money and his iPhone.

The boy claimed he was carrying the knife — which was never recovered — in self-defence and described the blade as “a little tool” with a 4 in blade.

But a police pathologist said the knife which inflicted the fatal wound was at least 5 in long and 2 in wide “like a kitchen knife”.

Mr Mirza said he did not think the teenagers were friends. He said: “Bilal was a good boy. He was handsome and everyone loved him, no one had a bad word about him. He was 18 and had his life in front of him, like a flower ready to bloom. This boy said he was friends with my son but I am his father and Bilal never talked about him.”

Mr Mirza, who lives with his eldest son Bader, 19, added: “It is very hard. I remember him but I cannot see him or hear him any more. I miss him so much. He was a very clever boy and wanted to go to university to learn how to fix computers. My life has been turned upside down by what’s happened and without Bilal I’m lost.

“Police should do more to stop young people carrying knives. The law should be tougher and there should be harsh sentences for those carrying knives and never let out of prison.”@1Ben_Morgan